FIG. 5 shows the structure of one known clutch and brake control system for bulldozers. The clutch and brake control system of this conventional structure is designed such that a steering lever 50, which also serves as a forward/reverse drive lever, is coupled to a steering control valve 53 integral with a brake valve through many rods 51, 52 and others, in order to operate right and left clutches and brakes. When slightly pushing the steering lever 50 down to the left (or to the right), the left (or right) steering clutch is brought into its half-disengaged state so that the vehicle gradually turns leftward (or rightward). When fully pushing the steering lever 50 down to the left (or to the right), the left (or right) steering clutch is brought into a completely disengaged state while the steering brake becomes actuated so that the vehicle turns quickly to the left (or to the right). A brake pedal 54 is coupled to the steering control valve 53 through rods 55, 56 and others. By depressing the brake pedal 54, only the right and left steering brakes are put in operation thereby stopping the vehicle.
The clutch and brake control system having the above conventional structure however reveals the following problem. Since the brake valve and the steering valve are integrally formed with the steering control valve 53, the system inevitably involves a complicated control linkage for coupling these integral valves to the steering lever and to the brake pedal, so that the looseness and deflection of the control linkage, and the like, hinder the vehicle from maintaining good operability.
The present invention is directed to overcoming the foregoing problem and therefore a prime object of the invention is to provide a clutch and brake control system for a tracklaying construction vehicle, the system enabling an improvement in the operability of the tracklaying construction vehicle by simplifying the control linkage extending from the operation section to the valves to reduce looseness and deflection.